It is common to heat fluids by the forced circulation of fluid through a heat exchanger which is in the combustion chamber of a furnace. Typically, the heat exchanger is in contact with the hot combustion gases emitted from one or more burners.
Normally, the housing defining the combustion chamber provides a relatively large heat conductive surface area exposed to the heated combustion gases. However, only a fraction of this surface area includes the heat exchanger. Thus, there usually are significant heat losses from the combustion chamber housing. Moreover, the efficiency of heat transfer to the fluid to be heated and the temperature of the combustion gases in the vicinity of the heat exchanger is reduced because of the relatively large volume and mass of air in the combustion chamber. The rate of heat transfer in such a system is determined, for a given heat exchanger, by the temperature differential between the fluid being heated and the hot combustion gases. The temperature increase of the fluid flowing through the heat exchanger is also dependent upon the rate of flow of the fluid through the heat exchanger.
Existing furnaces and heating boilers are typically designed and rated at a steady state combustion efficiency of approximately 80% (i.e. percent of energy available for heating due to the burning of fuel). The utilization efficiency or overall efficiency of these units is usually less than 80% under steady state firing conditions.
A change in ambient temperature of only a few degrees can be sensed by the human body. This is exactly what occurs when ordinary heating systems cycle on and off in an effort to maintain an average temperature in the heated space. The thermometers on ordinary thermostats are slow acting and have a tendency to display the "average temperature" of the space or room being monitored. It is common experience to experience a "chill" only to find that the thermostat displays the selected set point temperature (e.g. 70.degree. F.). In fact, if the thermostat could read the actual room temperature, it would be something less (e.g. 68.degree. F. or below). Due to the cycling of the heating system, this chilling sensation often occurs many times each hour.
Another problem is the household "juggler." This is the person who turns the thermostat up or down depending on how he or she feels at the moment. If the juggler is a little chilly, up goes the thermostat. If the juggler is too warm, either the thermostat is turned back down, or worse yet, the window is opened! Every time the heating system comes on and the room becomes warm, the juggler becomes "hot" and decides to lower the temperature. Then the heating system shuts off and the ducts, the furnace, and the flue which are hot, rapidly cool off and waste their heat. Now the juggler feels cold and turns up the thermostat. When the system comes on, part of the heat energy is "wasted" in heating up these components again. These losses are often called "startup losses". It represents lost efficiency in that the heat is not used to keep the room warm. Start-up losses have considerable effect on ones annual heating bill.
Thus, it is not so much a lower temperature that causes discomfort as it is the frequent and rapid temperature swings (e.g. about 3.degree. F. to 4.degree. F. up and down) such as occurs during the cycling of the heating system. Since heating plants or units are oversized for a vast majority of the heating season in most geographic locations, such furnaces have excess "standby capacity."
The standby capacity results from the fact that furnaces are typically sized to be effective during times of maximum design heat loss for a given geographic area. For example, Chicago, Ill. is rated at -10.degree. F. outdoor temperature and a 70.degree. F. indoor temperature--an 80.degree. F. temperature difference (T/D). However, such severe conditions are experienced for as little as 1% of the total heating season. Therefore, even assuming that the furnace is not oversized, it is evident that a steady state firing condition is achieved for less than 1% of the entire heating season. In other words, for 99% of the "normal" heating season in Chicago these heating units must cycle on and off to maintain the heated space (e.g. the interior of a building or dwelling) at a comfortable temperature.
Thus, at moderate temperature differentials (i.e., less than design value of 80.degree. F. in Chicago) between the inside and the outside air temperatures, the furnace raises the temperature from a low to high temperature faster than the ambient temperature of the room cools from the same high to low temperature. As the outdoor temperature moderates, the amount of standby capacity increases and the rapidity of low to high temperature swings increases. At colder outdoor temperatures the process is reversed and the rate of heating from a low to high temperature is slower than the rate of cooling from a high to a low temperature.
There is still another problem. During these on and off cycles a certain amount of heat is lost due to "reverse heat transfer." That is, when the furnace burner is off and the combustion chamber is hot, a "stack effect" takes place. Due to the relative difference in density between warm and cold air, air from the interior of the combustion chamber flows up the flue cooling the fluid (e.g., air) in the heat exchanger. These convective heat or energy losses are often referred to as "standby losses." Stand-by losses are accentuated by the conventional method of sizing furnaces and heating plants for the worst part of the heating season. Since steady state conditions do not exist for any appreciable length of time, (except maybe for the small percentage of the heating season when temperature differentials are equal to or greater than design conditions), standby and startup losses occur every time the heating plant is cycled on and off. Thus, as long as the furnace cycles, startup losses will occur at the beginning of each cycle and standby losses will occur at the end of each cycle.
From the foregoing it should be clear that the majority of existing furnaces are oversized. For example, if a home has a estimated heat loss of 80,000 BTU/hr. at design conditions, it is highly unlikely that a heating unit rated at 100,000 BTU/hr. input and 80,000 BTU/hr. output (80% combustion efficiency) will be used. More likely than not a larger unit, e.g., one rated at 120,000 BTU/hr. input and 96,000 BTU/hr. output or 125,000 BTU/hr. input and 100,000 BTU/hr. output, will be used. Most home heating units are produced with input ratings that differ by approximately 20,000 BTU/hr. with an input rating of at least 80,000 BTU/hr., (e.g., 100,000 BTU/hr., 120,000 BTU/hr., etc.). In other words, it is likely that the heating unit will be oversized by at least 20% to 25%. In some cases even larger units are used to be sure there is enough contingent capacity. Thus, in practice, heating plants in some buildings are oversized by at least 50%!
Currently, the most prominent method pursued for fuel savings is the "night setback" thermostat where the room temperature is reduced at least one time during the 24-hour day (i.e. usually at night or when the house is unoccupied during the day) by about 8.degree. to 10.degree. F. In order to achieve recovery from these setbacks, additional burner capacity of 20% to 25% must be used and must be available for use. It can be understood that if improperly sized, a conventional unit would not have the capacity to recover from a temperature lower than the outdoor design temperatures. For example, a properly sized unit having a 8.degree. to 10.degree. F. setback and a two hour recovery time, will only be efficient at one particular outdoor temperature. Thus, recovery will be faster at higher temperatures and slower at lower temperatures. Significantly, the recovery capability is completely gone as the outdoor temperature approaches its design temperatures.
One approach in attempting to offset resulting "excess capacity" inefficiences is to use heating units that have high and low firing rates. These units effectively are based on two design conditions (e.g., 80.degree. F. rise and 40.degree. F. rise). However, it is common knowledge among those skilled in the art that a burner has the greatest combustion efficiency at only one firing rate. Thus, if the burner is efficient at the high firing rate, it will not be as efficient at a low firing rate. Consequently, modulating or varying the firing rate is not a complete answer to the excess capacity problem.
Some have proposed to reduce standby heat losses by restricting the flow of warm air from the combustion chamber through the chimney. However, because draft affects the burning of the air and gas mixture in the burner, what might be gained by reducing standby losses is often lost by lower combustion efficiency.
It is well known that if a heating plant could be designed for 90% combustion efficiency, fuel consumption could be reduced by approximately 12 percent. However, for large residential burners, attempts to obtain anything greater than 80% combustion efficiency has resulted in incomplete combustion and corrosive moisture condensing in the flue. Even then the seasonal oversizing losses and the discomfort experienced by periodic high to low temperatures cycling would still exist. Thus, merely improving the efficiency of the burner or reducing standby losses are only partial measures to reduce wasted energy.
Few have recognized the wasteful practice of sizing furnaces for the worst set of design conditions anticipated and operating that furnace cyclically throughout the heating season. Geaslen U.S. Pat. No. 3,329,343 describes a heating system that uses a plurality of large water heaters or boilers connected in parallel to supply hot water for heating. Each of the boilers is actuated one at a time in direct response to the outdoor temperature. Each boiler has its own individual circulating pump and a set of inlet and outlet isolation valves. When the outdoor temperature decreases to a preselected value, the valves are opened, the pump is started and the boiler is placed in operation. Thus, the water stored within the individual standby boilers is not heated until it is needed. The boilers are shut off in response to the temperature of the circulating water exceeding a preselected set point.
Thus "room temperature", as such, does not cause the heating system to cycle. It should be clear that once the additional heat from the hot water is needed, it will take an appreciable amount of time to heat the mass of stagnant ambient water and for the heating system as a whole to have any effect. Such a heating system is inherently inefficient and wasteful because of the large mass of fluid that is stored at ambient temperature and because water has a high specific heat capacity.
Moreover, mixing the cold stagnant water with the already heated circulating warm water actually decreases the overall temperature of the water being circulated. Furthermore, start-up losses start out high and remain high until the bulk of water in the boiler is raised to the temperature required to maintain the room or the building warm. In addition, Geaslen's use of boilers, with the time required to heat the stored mass of cold water, would appear to preclude quick response and uniform temperature control.
Thus, while Geaslen shows some understanding of the problem of excess standby capacity, he has not really addressed or solved the problems of start-up losses and the maintainance of uniform room temperature while boilers are cycled on and off.
Van Vliet U.S. Pat. No. 3,935,855, describes a gas fired warm air furnace incorporating a plurality of flanking heat exchangers which are individually fired and across which a blower circulates air. In other words, a separate burner is provided for each heat exchanger and the heat exchangers are situated adjacent to one another and transversely to the flow of air being heated. Thus, air flows across all of the heat exchangers whether or not the corresponding gas burner is in operation. The gas burners themselves are fired automatically using a multistage thermostat. Since in this arrangement air is always circulated sequentially through and across all of the heat exchangers, this arrangement does not reduce standby losses from those heat exchangers where the gas burner is not being fired. This is because the temperature difference between the ambient and the circulating air is at a relatively high value. As a result, a percentage of the heat energy produced in the upstream burners "goes up the stack" in the idle downstream heat exchangers due to reverse heat transfer. Additional modules referred to in the patent will only increase such losses.
In spite of the apparent differences between the two patents, they are similar in that both provide for sequential heating of the medium being heated; both, while recognizing the problem of excess standby capacity, do no disclose an effective heating system to minimize start-up losses and standby losses; and neither patent discloses a system capable of maintaining the temperature of the space being heated generally uniform within a very narrow band.
What is needed is an innovative approach to heating system design that reduces start-up losses and standby losses, improves burner efficiency and substantially eliminates the wasteful effects of excess standby capacity.